As plastic molded articles requiring patterns of characters and/or symbols on their top faces and front side faces, there are generally many products, a representative example of which is a key cap. This key cap is used in a keyboard of, for example, a push-button telephone, a desk top electronic calculator, a typewriter, a word processor, a personal computer and the like. Hereinafter, reference will be made to the key cap as a plastic molded article for the description of the present invention.
Japanese Laid-open Patent Publication No. 58-155957 discloses a transfer printing method utilizing a transfer pad wherein a transfer sheet used for imparting a picture to various articles is used to form a pattern of characters and/or symbols on a key cap matrix surface by dyeing. That method uses a transfer sheet which is a support sheet on which a pattern of characters and/or symbols are formed by the use of an ink containing thermally migrateable dyes. The transfer sheet is placed on a top face of the key cap matrix with the pattern held in contact therewith. The transfer pad is then pressed from the side of the support sheet of the transfer sheet to cause the pattern to tightly contact the surface of the key cap matrix. A heating treatment is subsequently effected while the key cap matrix surface and the pattern are held in tight contact with each other to cause the thermally migrateable dyes within the pattern to be diffused thermally to form a desired pattern of characters and/or symbols on the surface of the key cap matrix. The support sheet is thereafter removed. Since the pattern of characters and/or symbols is formed by dyeing, this method is effective to provide a product excellent in resistance to wear as compared with a so-called direct printing method and is also effective to provide an inexpensive product as compared with a so-called bicolor molding method, and has, therefore, an extreme utility.